A back-illuminated image sensor is known wherein an incident beam, such as visible light, is irradiated to one face of a chip (back face) opposite to the other face (surface) of the chip on which electrodes and the like are disposed (see Japanese Patent Application Laid-Open Publication No. H9-331052, for example). The back-illuminated image sensor is provided with a converting portion for each pixel (a photoelectric converting portion in the case that the incident beam is visible light) on the back face side of the chip, and it is provided with portions for processing signal charges (charge processing portions) in some way, such as an A/D converter and a signal storage portion on the surface side of the chip.
Since the back-illuminated image sensor can achieve a high aperture ratio of nearly 100%, it can realize very high sensitivity. For this reason, the back-illuminated image sensor is frequently used in applications requiring high sensitivity such as fields of astronomy and electronic microscopy. Furthermore, due to its high sensitivity, the back-illuminated image sensor is also suited for high-speed image capturing wherein exposure time for each one of images is short.
Mixture of signal charges into the charge processing portions is the major problem in the back-illuminated image sensor. More specifically, signal charges, such as photoelectrons, generated in the converting portion, are mixed by diffusion or diffraction in some parts of the charge processing portion into which the signal charges should not be mixed intrinsically. The mixed signal charges hamper the function of the charge processing portion.
Transmission of light is another problem in the back-illuminated image sensor. The chip of the back-illuminated image sensor is required to be made as thin as possible. If the chip is thick, before the charges generated corresponding to the incident beam reach the function region, the charges in the adjacent pixels are mixed mutually, and noise owing to crystal defects inside the chip is mixed into the signal charges. Since the chip is thin as described above, light having long wavelengths and high transmittance (low absorption coefficient) reaches the function region on the side of the surface, resulting in that undesirable and unnecessary charges are generated inside the function region. These charges also hamper the functions of the elements provided in the function region.
The present inventors have developed an in-situ storage image sensor (ISIS) provided with linear signal storage portions inside or in the vicinity of pixels. For example, the following documents have been issued. Japanese Patent Application Laid-Open Publication No. 2001-345441; Takeharu ETOH et al: “A CCD Image Sensor of 1M frames/s for Continuous Image Capturing of 103 Frames,” Digest of Technical Papers, 2002 IEEE International Solid-State Circuits Conference, 2002, Vol. 45, p. 46 to 47; and Takeharu ETOH and four others: “An In-situ Storage Image Sensor of 1M frames/s with Slanted Linear CCD Storage,” Journal of the Institute of Image Information and Television Engineers, the Institute of Image Information and Television Engineers, 2002, Vol. 56, No. 3, p. 483 to 486. In the case that a back-illuminated structure is adopted for the in-situ storage image sensor, the above-mentioned problems owing to the mixture of signal charges and the transmission of light become particularly significant.